Right-Biassed Crystalline Lens Asymmetry in the Thornback Ray (Rajiformes: Rajidae: Raja clavata): Implications for Ocular Lateralisation in Cartilaginous Fish

Directional asymmetry (DA) is a widespread yet often overlooked feature of animal morphology. Here, we report a consistent right-biassed asymmetry in the crystalline lenses of the thornback ray Raja clavata. Across 71 individuals sampled from the Strait of Sicily, 24% exhibited lens asymmetry, and in all cases, the right lens was larger. This pattern, supported by binomial and distributional tests, represents the first evidence of ocular DA in this species. Body size and age emerged as the primary correlates of asymmetry: the odds of exhibiting DA increased significantly with body weight, whereas the effects of sex and sampling site were not significant. The prevalence of asymmetry thus appears to rise with age and ontogenetic growth. Two non-exclusive mechanisms may account for this pattern. First, the association with positive allometry (b = 3.33) suggests that right-lens enlargement could reflect a functional or developmental lateralisation, potentially conferring a visual or ecological advantage to larger individuals. Alternatively, the same right-lens bias could arise through an age-related pathological process, such as oxidative or osmotic lens swelling preceding cataract formation, consistent with asymmetric physiological wear. These findings reveal a novel case of morphological lateralisation in an elasmobranch and highlight the need for comparative, histological, and functional approaches to disentangle adaptive asymmetry from lateralised senescence in the visual system of R. clavata.